Many processes have been developed to capture gaseous pollutants, such as carbon dioxide, sulphur dioxide and nitrogen oxides (NOx) from industrial process streams. One such method comprises the use of an absorbent to selectively absorb one or more gases from a feed gas stream. For example, U.S. Pat. Nos. 5,019,361 and 5,262,139, the disclosure of each of which is hereby incorporated by reference, each disclose an absorbent for recovering sulphur dioxide from a waste gas stream. Co-pending U.S. patent application Ser. No. 10/459,519, the disclosure of which is incorporated herein by reference, discloses an absorbent for absorbing carbon dioxide from a waste gas stream. Co-pending U.S. patent application Ser. No. 10/902,134, the disclosure of which is hereby incorporated herein by reference, discloses an absorbent for recovering NOx from a waste gas stream.
According to Zhao et al, Hg Absorption in Aqueous Permanganate, AlChE Journal, Vol. 42, No. 12, December 1996, pp 3559-3562, aqueous acidic permanganate has been known to be an effective solvent for elemental mercury vapour since 1956. Zhao et al discloses that aqueous acidic permanganate is also a suitable absorbent for NO. However, except for use the use of acidic permanganate impinger solutions to capture mercury vapor for analyzing gas samples, to the knowledge of the inventors, no commercial process has been developed that utilizes permanganate,
Permanganate has been used in absorption processes as corrosion inhibitors. For example, U.S. Pat. No. 4,440,731 (Pearce) relates to corrosion inhibiting compositions for use in aqueous absorbent gas-liquid contacting process, and in particular, the removal of carbon dioxide from industrial combustion gasses using a solution containing an alkanol amine absorbent. The corrosion inhibiting compositions are used to reduce degradation of the absorbent under the conditions at which the absorbent was used and corrosion of the metals in contact with the absorbent solution during use. The corrosion inhibiting compositions comprise at least above 50 ppm of copper+2 and at least from 50 parts per million parts of solution one or more of dihydroxyethylglycine, an alkali metal carbonate, an alkali metal or ammonium permanganate and nickel and/or bismuth oxides which are added to the absorbent solution.
Different processes have been developed for the removal of mercury from flue gasses. For example, U.S. Pat. No. 6,719,828 (Lovell et al) discloses the use of a phyllosilicate substrate, for example vermiculite or montmorillonite, which acts as an inexpensive support to a thin layer for a polyvalent metal sulfide. The sorbent is prepared by ion exchange between the silicate substrate material and a solution containing one or more of a group of polyvalent metals including tin (both Sn(II) and Sn(IV)), iron (both Fe(II) and Fe(III)), titanium, manganese, zirconium and molybdenum, dissolved as salts, to produce an exchanged substrate. Controlled addition of sulfide ions to the exchanged silicate substrate produces the sorbent. The sorbent is used to absorb elemental mercury or oxidized mercury species such as mercuric chloride from a flue gas that contains oxidizable gases (e.g., SO2, NO, NO2, and HCl).
U.S. Pat. No. 6,790,420 (Breen et al) discloses a process for the removal of mercury from combustion flue gasses using oxidation. As stated in the abstract of Breen et al, “ammonia and optionally carbon monoxide are injected into the flue gas in a manner so that there are sufficient amounts of these materials in the flue gas, when the flue gas is at a temperature of from 900° F. to 1350° F., to oxidize the metals within the flue gas. The oxidized metals are then attracted to particulates present in the flue gas. These particulates bound with oxidized metals are removed from the flue gas by a particulate removal device such as an electrostatic precipitator or baghouse”.
United States Patent publication 2002-0174646 (Sanders) discloses a method for reducing NOx and/or CO/CO2 emissions from the combustion of coal products or hydrocarbons. One or more curtains of particulate elemental iron, in the form of flakes or the like, are fanned across the combustion chamber and/or the exiting flue gas stream while it is still at high temperature. In a coal-fired furnace or boiler, particulate magnetite can be injected e.g. into a region just above the coal which will effectively sequester Hg and heavy metals at a location below a “rag layer” combustion zone in the vicinity of which iron is injected. Some iron may also adsorb mercury via “red iron” formation and be removed along with solid oxides, iron carbide and siderite via the ash chute of a boiler. Any particulates carried downstream in the flue gas stream are removed, prior to reaching the stack, by cyclones, scrubbers, precipitators or the like.
U.S. Pat. No. 5,482,536 (Ngai et al) relates to an apparatus for containing and scrubbing toxic or corrosive gases from a leaking pipe or cylinder utilizing a scrubbing media which, when it contacts the leaked gas, cleans or removes the harmful component, thus allowing release to the atmosphere of the cleaned air. In the case of a hydride gas, for example, arsine, germane, phosphine, hydrogen sulfide, hydrogen selenide, phosphine or organometallic mixtures, such as dimethyl zinc or diethyl telluride, the scrubbing media is preferably a high surface area carbon impregnated with oxides of copper or aluminum impregnated with potassium permanganate.
One disadvantage with the use of permanganate is that it is not regenerable once it has been used to capture mercury. Further, permanganate will absorb other oxidizable gases (i.e. NO or SO2) in a combustion gas stream.